I guess you have to deal in upper and lower bounds for this type of question. The upper bound is hard to answer and likely doesn't have an answer other than "the maximum I could reasonably own without people finding out". I would pay a lot of money to keep my anonymity. I have a feeling this would be far less than 1 billion euro.
The lower bound is easier and more interesting, I would say take a lump sum that would pay off my mortgage with enough left over such that when it is invested in a mutual index fund would pay around 100K euro per year after tax at 3% return. I believe that would allow the main fund to grow slightly faster than inflation (so i am not losing value). So around 3 million give or take? Obviously you need more if you aim for a 2 or 1% payout rate per year.
The goal would be to fund my current or slightly improved lifestyle without needing to work for a living. Freeing me to do what is most meaningful to me, rather than working 40+ hours a week. I like my job, but not working 40 hours per week for the next 30 years is like being gifted 10 years of do-what-you-want-to-do time. And time is all you have when you get right down to it.